1. Field of the Invention
The present invention relates to a dampening volume control apparatus for offset press, more specifically, as to an apparatus for setting desired values of dampening volume.
2. Description of the Prior Art
In offset printing, phenomena so called scumming or similar problems that ink is oversupplied even to non-imaging areas of printing papers occurs when less amount of dampening solution relative to that of the ink is supplied to an offset plate. On the contrary, occurrence of emulsification including water streak and uneven density of printing are observed when too much amount of the dampening solution relative to that of the ink is supplied to the offset plate. So that, it is necessary to maintain the amount of both the ink and the dampening solution supplied to the offset plate within an appropriate range during printing work.
In order to prevent both the phenomena and the emulsification, a dampening volume control apparatus shown in FIG. 3 which keeps supply amount of the dampening solution in a certain volume is proposed (see Japanese laid-open publication No. Hei 4-83640). In the control apparatus, a desired value of the dampening volume (a supply amount of the dampening solution) is stored in a controller 123, then actual dampening volume of the dampening solution supplied to a plate 33 is detected by a dampening volume sensor 41. The detected volume of the dampening solution is compared with the stored value. Revolution speed of a motor 25 is controlled by the controller 123 in accordance with a result of the comparison. In this way, revolution speed of a water fountain roller 28 is varied. So that, feed-back control is carried out so as to make the dampening solution supplied to the plate 33 coincide with the desired value stored in the controller 123 as a result of varying the revolution speed of the water fountain roller 28.
However, the dampening volume control apparatus described earlier has following problems to be resolved. Quality of printing can be controlled by storing the desired value in the controller 123 when the desired value is determined as a certain value. Nevertheless, the desired value itself is varied by following factors such as room temperature, humidity at the room, material of the plate and others. Actually, the desired value is determined by an operator of the offset press under trial and error bases by referring the quality of the printing. In a concrete form, the desired value is adjusted in accordance with the quality of the printing done on the printing papers by carrying out following procedures. A desired value is set for temporary purpose, and then printing work is carried out until a volume of the dampening solution supplied to the offset plate becomes a stable condition as shown in FIG. 4. The procedures described in above need to be repeated. As a result, it is required both a certain period of time and printing papers to determine a desired value appropriate for printing in a good condition.
The time and the printing papers consumed during the adjustment occupies a large amount of total resources, especially in a small lot printing carried out recently.